Multi-channel optical signal transmission systems typically monitor and control transmitted signal power level in each channel in order to correct for effects such as temperature change and device aging. Conventional monitor and control techniques are usually implemented locally at the transmitter and performed independently on each channel. For example, each of the channel signal transmitters in a multi-channel wavelength division multiplexed (WDM) laser transmitter may include a photodetector which monitors the rear facet light emitted by a semiconductor laser used to generate the corresponding channel signal. The photodetector output is then used in a feedback loop to control the output optical signal power level of the channel signal transmitter.
This conventional built-in independent channel control approach requires a separate optical detector and feedback control loop for each channel, and therefore increases the cost and complexity of the optical system. Moreover, integrated multi-channel laser transmitters may not be able to utilize such built-in independent channel control techniques, and may therefore require alternative techniques such as external detection. For integrated distributed feedback (DFB) laser arrays or integrated multi-frequency lasers, conventional techniques for monitoring the WDM channels may involve using an optical spectrometer to separate the channels or superimposing unique modulation tones over the data. Unfortunately, these external detection techniques also unduly increase the cost and complexity of the channel signal power control process.
It is therefore apparent that a need exists for an improved technique for stabilizing or otherwise controlling optical signal power levels in multi-channel optical signal transmitters.